This invention relates to a device for determining geometrical defects in a vehicle wheel.
A vehicle wheel, i.e. the combined tyre plus wheel rim, can present shape imperfections or defects which reduce the running comfort of the vehicle, these being due partly to defects in the tyre construction and partly to defects in the wheel rim construction.
The said shape defects can be largely compensated on mounting the tyre on the wheel rim by suitably positioning the tyre relative thereto.
The same applicant has invented an apparatus able to determine geometrical defects of the wheel when this is locked on the self-centering unit of a tyre removal machine.
This apparatus forms the subject of the European Patent Application published under No. 1,054,247, to which reference should be made for a more detailed description.
Although said apparatus performs the functions for which it is proposed, it has proved inaccurate in determining imperfections.
A first inaccuracy is due to the fact that the centre of the wheel rim, or of the wheel, is calculated mathematically by interpolation, involving an inevitable measurement error which is then reflected in the determination of the results.
In addition, the eccentricity defects of the wheel rim are measured at the inner edge of the wheel rim and not, as would be correct, at the channel which receives the tyre bead. This introduces into the measurement an approximation which is often unacceptable.
The object of the present invention is to overcome the drawbacks of the said apparatus within the framework of a rational and reliable solution.
The invention attains said object by providing, for determining the geometrical defects of a vehicle wheel rim or wheel, a device provided with means for mechanically determining the centre of the wheel rim and/or wheel. This advantageously results in complete elimination of the error in determining the wheel rim centre which arises in the known apparatus.
The device of the invention comprises, for measuring the eccentricity of the wheel rim and of the wheel, a device to be positioned in contact alternately with the edge of the wheel and with the edge of the wheel rim of which the eccentricity is to be measured, an angular sensor for measuring the angular position of said measurement unit about a fixed point, and means for associating and processing the data measured by said unit and by said angular sensor to determine the angle through which the tyre must be rotated on the wheel rim to compensate their errors of shape.
In particular, according to the invention, said angular sensor comprises a movable part associated with said eccentricity measurement unit, and a fixed part rigid with a shaft provided with self-centering support means which enable it to be positioned exactly coaxial with the axis of said wheel rim.
Finally, it should be noted that the invention can be advantageously associated with the self-centering plate with which tyre removal machines of known type are provided.
Further characteristics of the invention are defined in the claims.